1. Field of the Invention
This invention relates to a thermal recording element for recording information by exposure to high intensity radiation to thermally deform the recording layer and more particularly it relates to a high-sensitive thermal recording element capable of recording even by a low power high energy density beam.
2. Description of the Prior Art
As recording elements for recording information by exposure to high intensity radiation such as a laser beam, etc., there is a thermal recording element in addition to light-sensitive elements using silver salts, etc. In the thermal recording element the recording layer has a high optical density and the recording layer produces a local temperature increase by absorbing a high intensity irradiation whereby the irradiated portions are removed by thermal deformation such as fusion, evaporation, aggregation, etc. Information is recorded by the difference in optical density between the irradiated portions and non-irradiated portions.
Such a thermal recording element has the advantages that processings such as development, fixing, etc., are unnecessary, a dark room is unnecessary since the thermal recording element is not sensitive to ordinary room light, high contrast images are obtained, and addition recording (add on) of information is possible.
Recording on such a thermal recording element is generally performed by converting recording information to an electric time-succession signal and scanning the recording element by a laser beam the intensity of which is modulated according to the electric signal. In this case, there is also the advantage that a recording image is obtained in real time.
As the recording layer for such a thermal recording element, metals, dyes, plastics, etc., are suitably used and in general an inexpensive material can be used for the purpose. Such recording elements are described in, for example, M. L. Levene et al, Electron, Ion and Laser Beam Technology, (the records of the 11th Symposium held in 1969); Electronics, p. 50 (Mar. 18, 1968); D. Maydan, The Bell System Technical Journal, Vol. 50, 1761 (1971); C. O. Carlson, Science, Vol. 154, 1550 (1966), etc. Some of these recording elements contain metal recording layers. For example, such a recording element comprises a support having formed thereon a thin layer of a metal such as Bi, Sn, In, etc., and has excellent properties as thermal recording element since an image of high resolving power and high contrast can be recorded.
However, the recording element having a thin metal layer generally exhibits light reflectance of higher than 50% to laser light used for recording, which makes it impossible to effectively utilize the energy of the laser beam and hence the power of the laser beam used for recording must be higher, which requires a laser light source of high output for recording at high scanning speeds, and thus the recording apparatus used becomes larger and expensive.
Various recording elements having high recording sensitivity have been studied and as an example a recording element of three layer construction composed of Se, Bi and Ge is disclosed in U.S. Pat. No. 3,560,994. A thin layer of Ge is formed on the layer of Se and Bi to reduce the light reflectance of a thin layer composed of Se and Bi. However, the use of Se, etc., is undesirable since there is a possibility of toxicity problems and the images recorded are not satisfactory.
As another example of a recording element having a reflection preventing layer, a recording element having on the metal layer a reflection preventing layer having light absorption in the wavelength region of the laser beam used for recording is disclosed in Japanese Patent Application (OPI) No. 74632/76 (The term "OPI" as used herein refers to a "published unexamined Japanese patent application"). However, even if a reflection preventing layer is formed it is very difficult to completely eliminate the occurrence of light reflection and even if the occurrence of light reflection can be completely eliminated, a laser light source of high power is required to obtain thermal deformation of a thin metal layer by the irradiation of laser light and a recording element having higher sensitivity is needed.
Since the recording layer, e.g., a thin metal layer of the thermal recording element as mentioned above, is liable to scratch, a protective layer is formed on the metal layer of the recording element to improve the durability, mechanical strength, time stability, etc., of recording element. The protective layer must transmit the light beam of high energy density used for recording, have high mechanical strength, not react with the recording layer, exhibit good coating property, etc.
As the materials for the protective layer, inorganic materials or organic materials may be used. As the inorganic materials, transparent materials such as Al.sub.2 O.sub.3, SiO.sub.2, SiO, MgO, ZnO, MgF.sub.2, CuF.sub.2, etc., have been used. Inorganic materials for the protective layers are an excellent means and they are disclosed in, for example, Japanese Patent Application (OPI) Nos. 96716/74, 59626/76, 75523/76, 88024/76 and 134633/76. However, these recording elements having protective layers possessing effective strength as described in the above patent applications suffer from the drawback that the recording sensitivity is greatly reduced as compared with that of a recording element having no protective layer.